I'm having trouble making HiDPI work on my MacBook Pro (13" 2015 with Retina, Mojave up-to-date).
I plugged it into a 1440p Dell U2719D, which works fine out of the box, except that fonts look ugly (too small) and the UI is small as well. I tried enabling HiDPI resolution, for example 1280x720 (perfect 2 for 1).
Here are my findings:
HiDPI resolutions are simply not available in the system's menu in this setup. I tried the hack (for example here), but there is no way to make them appear in the system's Display Preferences. 720p is listed there but it's not HiDPI and looks ugly.
Using RDM, I'm able to see many more resolutions, including HiDPI resolutions. However, among HiDPI resolutions, only 16:10 resolutions work (1680x1050, 1440x900, 1280x800 etc.). For all other, HiDPI, non 16:10 resolutions, I click on them in RDM, nothing happens. (I also tried other resolution managers with the same effect).
It's worth noting that the 13" built-in display is 16:10 (at 3360x2100 native, 1280x800 scaled). 1280x800 looks fine but the black borders are weird (and it's not perfect 2 for 1 pixels).
Is this behaviour normal? Is this a bug or lack of feature? Is there a workaround?
After some careless tinkering (notably using this link), the HiDPI resolution "1281x720" appeared (!) which actually works (!!). It's not perfect but it looks sharp and uses most pixels on the display.